1. Field of the Invention
This invention relates to a circuit of selecting reference voltage, and more particularly to employ a compare circuit to decide a state of an input signal, and a reference voltage select circuit selects a suitable voltage according to the state of the input signal.
2. Description of the Prior Art
A dynamic random access memory (DRAM) decides upon a logical signal of an input signal according to a voltage of the input signal. When a system outputs a logical signal xe2x80x9c1xe2x80x9d, a voltage of the logical signal xe2x80x9c1xe2x80x9d is an input high voltage VIH; when the system outputs a logical signal xe2x80x9c0xe2x80x9d, a voltage of the logical signal xe2x80x9c0xe2x80x9d is an input low voltage VIL.
In some applications, for example, a low power synchronous dynamic random access memory (LPSD), a signal voltage is an input high voltage or an input low voltage that is determined by a system voltage. If a ratio of the signal voltage and the system voltage is larger than 0.8, the signal voltage is determined as the input high voltage VIH, and if the ratio of the signal voltage and the system voltage is smaller than 0.2, the signal voltage is determined as the input low voltage VIL. In the DRAM, a logical signal is the input high voltage VIH or the input low voltage VIL, that is determined according to a reference voltage VREF, it is determined voltage that. If the input signal is higher than the reference voltage VREF, it is determined as xe2x80x9c1xe2x80x9d; and if the input signal is lower than the reference voltage VREF, it is determined as xe2x80x9c0xe2x80x9d. In general, the reference voltage VREF is an average of the input high voltage VIH and the input low voltage VIL for determining the logical signal under an optimum state due to a toleration for a variation resulting from a noise being equal between the input high voltage VIH and the input low voltage VIL.
Different systems have respective different system voltages and so the input high voltage VIH, the input low voltage VIL and the reference voltage VREF are changed according to the system voltage. For example, a system voltage of first system is 2.5V, an input high voltage VIH thereof is 2.0V, an input low voltage VIL thereof is 0.5V and a reference voltage VREF is 1.25V; a system voltage of second system is 1.8V, an input high voltage VIH thereof is 1.44V, an input low voltage VIL thereof is 0.36V and a reference voltage VREF is 0.9V. Therefore, a design of a different reference voltage is needed for suiting to a different system. In the conventional arts, a reference voltage circuit with a fuse can change a reference voltage of the circuit. For example, if the fuse is unfused, the reference voltage provided by the circuit is suitable for the first system voltage 2.5V, and if the fuse is fused, the reference voltage provided by the circuit is suitable for the second system voltage 1.8V. But the circuit needs an extra process of fusing the fuse to change the reference voltage thereof and further the circuit only suits with the system voltage 1.8V when the fuse is fused.
Hence, aimed at the aforementioned problem of the conventional arts, the present invention employs a design of a select reference voltage circuit to select different reference voltages for suiting with different systems having different system voltages.
In the conventional arts, a reference voltage circuit is only applied in a system having a special system voltage, or the reference voltage circuit with a fuse needs the extra process of fusing the fuse and the circuit only suits with a system having a special system voltage regardless of the fuse is fused or unfused. It is an objective for present invention to provide a select reference voltage circuit for a logical electrical system to select a suitable reference voltage according to a system voltage.
It is another objective for present invention to employ a select reference voltage circuit for a logical electrical system to determine a state of a system voltage, and a reference voltage select circuit can select a suitable reference voltage according to the state.
It is a still another objective for present invention to provide a select reference voltage circuit for a logical electrical system to determine a suitable reference voltage for a logical system only during a short time after the logical system turns on. Hence, the determined reference voltage is not changed under an unstable system voltage.
It is another objective for present invention to provide a select reference voltage circuit for a logical electrical system that turns off after a suitable reference voltage is determined for economizing on consumption power.
As aforementioned, the present invention provides a select reference voltage circuit for a logical electrical system comprising a compare circuit and a reference voltage select circuit. The compare circuit receives both a system voltage and a base voltage of a logical electrical system and compares the system voltage with the base voltage to output a compare signal. The reference voltage select circuit receives the compare signal and selects a reference voltage coupling with the compare signal to output to the logical electrical system. Therefore, the device can select a suitable reference voltage for a different logical electrical system to output. A square wave generate circuit is further employed to receive a turn-on signal from the logical electrical system and generate a pulse signal. The compare circuit and the reference select circuit turn off when receive a turn-off signal of the pulse signal dropping down for avoiding changing the determined reference voltage due to a unstable system voltage. Moreover, a consumption power of the circuit can be efficiently economized.